1. Field of the Invention
The present invention relates a device to prevent damage to printing groups of web-fed rotary presses that print webs fed by a cylinder which is placed on a reel changer. A fault detector is provided that has deflection devices next to both web edges that can create a deflective force transverse to the plane of web transport that can be overcome by the tension of the web during normal operation, and a monitor is provided that detects changes.
2. Background Art
There is a prior-art device of this kind disclosed in EP 429 970 B1. This prior art arrangement relates to a web-break detector which follows the printing group and triggers a web catching device between the last printing group and the drier. This prior art device is based on the fact that the web tension is released when the web tears; this causes the web to be displaced which is detected by the monitors. The monitors are designed as light barriers whose optical axis is angled in the direction of the width of the web and is interrupted at the edge of the web during normal operation. A light barrier only registers "yes" or "no." This is sufficient for a web-break switch following the printing groups. Experience has shown, however, that the web entering the printing groups can have lateral defects arising especially from faulty gluing at the connection between the old and new web and/or transport damage which can produce strips wound on one side of a printing group roller. To deal with this, it would be insufficient to place the prior art arrangement in front of the printing groups. The size of the defect passing through the system cannot be determined with the prior-art arrangement. It would hence also be impossible to permit a certain tolerance width. This would in certain circumstances lead to incorrect reactions. It would also be impossible with the prior-art arrangement to evaluate the type of fault passing through the system from the signals generated by the monitors so that an adequate reaction often cannot occur. A further disadvantage is that due to the arrangement of the light barriers, improper adhesion between overlapping web ends during reel exchange cannot be recognized. The prior-art arrangement would therefore not be suitable to prevent damage to printing groups arising from adhesion faults, etc.